A $0 GRPO can severely affect your inventory value, especially if that item is consumed before the A/P Invoice corrects it. Below is a simple query to utilize as an alert that can help notify a user when a GRPO document is created without a value.
SELECT T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T0.[NumAtCard] FROM OPDN T0
INNER JOIN PDN1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T1.[LineTotal] = '0' AND T0.DocDate=cast(GetDate() as DATE) AND T0.[DocStatus] = 'O'